I. Technical Field
The invention relates to an information recording medium capable of three-dimensionally recording a recording pit and an optical information recording/reproducing apparatus for recording information to and reproducing information from this medium. Particularly, the invention relates to an information recording medium and an optical information recording/reproducing apparatus capable of reproducing information from recording layers at satisfactory SN ratios.
II. Description of Related Art
Optical disks such as CDs (compact disks) and DVDs (digital versatile disks), optical card memories and the like are used as optical information recording media.
Prior Art (Yoshimasa Kawata et. al.: “Three-dimensional optical memory using an organic multilayered medium”, Optics Japan 2000 Extended Abstract, pp 95-96 (2000)) discloses an information recording medium in which a plurality of recording layers 101 are three-dimensionally laminated as shown in FIG. 8 in order to realize the larger capacity of recorded information.
This information recording medium 114 includes a recording portion 103 atop a transparent substrate 109 made of glass, wherein recording layers 101a to 101d made of an urethane-urea copolymer material, which is a photon-mode recording material, and intermediate layers 102a to 102c made of a PVA (polyvinyl alcohol) film and a PMMA (polymethyl methacrylate) film are alternatively laminated.
In the information recording medium 114, a pulsed laser beam having a pulse width of about 100 femtoseconds and a high peak power is, for example, focused as a recording light 107 on the desired recording layer 101c by an objective lens 106 to record information. When the recording light 107 is focused on the recording layer 101, absorption occurs as if the wavelength of an irradiated light became a half only at a portion (focal point) with a high power density of the light by the two-photon absorption process, which is one of nonlinear absorption phenomena, whereby a recording pit 105 is written. Accordingly, the attenuation of the recording light can be suppressed since the two-photon absorption occurs only at the focal point even if there are a lot of recording layers, wherefore sufficient recording can be carried out even at the lower recording layers.
On the other hand, upon reproduction, a recording light 107 with low power is focused at the recording pit 105, and a reflected light 108 for reproduction is detected by a light detector (not shown) via the objective lens 106, whereby a signal is reproduced.
Since a plurality of recording layers 101 in which the recording pit 105 is recorded are three-dimensionally laminated in a direction of an optical axis (z-axis direction) of the objective lens 106 as described above, three-dimensional recording can be carried out in the recording portion 103, thereby increasing the information capacity.
However, the conventional information recording medium 114 whose information capacity is increased by three-dimensionally laminating a plurality of recording layers in the optical axis direction (z-axis direction) cannot provide a sufficient intensity of the reflected light 108 for reproduction if the number of the recording layers is large. Specifically, the attenuation of the light in the optical axis direction is increased if the number of the recording layers is large, wherefore no sufficient reflectivity of the reproducing light can be obtained in the lower recording layers. Since an increase in the reflectivity of the recording light at the recording layers leads to a reduction in the transmission amount of the reproducing light, the reproducing light cannot be sufficiently irradiated to the lower recording layers if the reflectivity of the reproducing light at the upper recording layers is increased. Thus, it is difficult to increase the reflectivity at the recording layers in order to increase the intensity of the reflected light 108 for reproduction from the recording pit 105. In the conventional information recording medium capable of three-dimensional recording, the reflectivity of the reproducing light was, for example, a small value of 0.1 to 1%.
Unlike CDs, DVDs and like disks, no metal reflection film is formed between the transparent substrate 109 and the recording portion 103 in this type of information recording medium capable of three-dimensional recording. This is for the following reason. Since the reflectivity of the reproducing light is low in the information recording medium capable of three-dimensional recording, if a light having transmitted through the recording portion is reflected by the metal reflecting film, it turns into stray light to reduce an SN ratio of the reproduced signal.
Accordingly, part of a transmitted light 110 having transmitted through the recording portion 103 provided with a plurality of recording layers 101 turns into various reflected lights 111 from a substrate side as shown in FIG. 8. For example, a light 110a having about 80% of the power of an incident light enters the transparent substrate 109. Then, a light having about 96% of the power of the light 110a (77% of the incident light) turns into a transmitted light 112a to transmit in the air, but the remaining light of about 4% (3% of the incident light) is reflected by an underside 113 of the transparent substrate 109 (reflected light 111a). Part of the reflected light 111a has a high possibility of passing through the recording layers 101 to be detected by the light detector via the objective lens 106. Since the reflected light 111a from the substrate contains no information, it turns into stray light that is so-called noise light. Accordingly, the reflected light other than the expected reflected light from the medium is detected as the stray light by the light detector, and the SN ratio of the reproduced signal tends to decrease since an amount of the reflected light 108 for reproduction from the recording pit 105 is small in the information recording medium 114 capable of three-dimensional recording as described above.
Out of the light having passed through the recording layers 101, for example, an oblique light 110b also enters the transparent substrate 109. The oblique light 110b is reflected by the underside 113 of the transparent substrate 109 to turn into a reflected light 111b from the substrate side, part of which is refracted, diffracted or multiply reflected by the recording pit 105 in a plurality of recording layers 101 to change its advancing direction, and is detected as stray light by the light detector via the objective lens 106. Such stray light tends to reduce the SN ratio of the reproduced signal.
Further, the incident light having passed through the recording portion 103 is, for example, partly reflected at an interface between the recording portion 103 and the transparent substrate 109 to create a reflected light 111c similar to the reflected light 111b. Since this reflected light 111c from the substrate side contains no signal as well, it turns into stray light.
Furthermore, the transmitted light 112a having transmitted through the medium advances toward the optical information recording/reproducing apparatus facing the medium. Part of this transmitted light 112a enters the medium again by being reflected by a surface of the apparatus facing the medium, and detected as a stray light.